For the past several years, many crude oil refiners in North America have experienced greater than anticipated equipment surface fouling of their refining units. Because the increased fouling rate dictates more frequent shutdowns for cleaning, the net business result is a significant increase in the overall cost of operation.
It is believed that a primary source of the fouling is metal soaps of partially esterified phosphate complexes typically present in small, but measurable, amounts (usually 5,000 ppm or less) in the crude oil as a result of being injected into oil wells during hydrocarbon-based hydraulic fracturing operations.
When an oil well is fractured using hydrocarbon fluids, a three-component mixture is simultaneously injected. The mixture contains 1) an organic liquid, such as diesel or kerosene, 2) proppant particles, for example, sand and 3) a reversible viscosifier package for the organic liquid, typically containing a metal soap of a partially esterified phosphate, as well as a rate-controlled viscosity reducer generally referred to as a "breaker."
The mixture is injected into the well at a pressure greater than the natural oil well formation pressure and also high enough to result in the fracturing of the formation.
Fracturing the well produces channels in the rock so that oil flow can be maximized. Use of an organic liquid during fracturing serves to reduce problems associated with water-sensitive formations. The proppant helps to hold the fractured formation open and provides a stable porous medium for greater oil flow. The use of a reversible viscosifier, such as a metal soap of a partially esterified phosphate (MSPEP), serves to thicken the fluid, thereby uniformly suspending the proppant during injection, facilitating proppant placement and finally reducing the liquid's viscosity through a controlled acid-base reaction of the partially esterified phosphate with the breaker. MSPEPs are extremely effective in hydrocarbon-based hydraulic fracturing operations and are generally regarded as the preferred viscosifiers for this application.
The large increase in fouling rate observed over the past few years at refineries is attributed mainly to the increase in the number of oil wells fractured with these types of hydrocarbon-based phosphates. As mentioned, this has become a major concern for North American refiners as it has increased their overall operation costs due to more frequent cleaning of the refining equipment. That is, while the fouling problem resulting from the use of MSPEPs has probably always occurred, the increased severity of the fouling has only recently been seen due to an increase in the number of wells fractured with hydrocarbon-based fracturing fluids containing MSPEPs.
The fouling deposition problem resulting from the use of MSPEPs is particularly severe because the preferred cleaning is a combination of mechanical and chemical methods. The use of such methods is expensive because refinery distillation units must be shut down to carry out the cleaning, thereby leading to lost refining production.
Accordingly, it would be desirable to provide a method of reducing the concentration of MSPEPs from a hydrocarbon flowback fluid resulting from hydrocarbon-based hydraulic fracturing operations. A "hydrocarbon flowback fluid" is generally defined as the hydrocarbon fracturing fluid used in the fracturing operation, initially injected into the oil well during fracturing, and later "flowed back" out of the well as a natural consequence of returning the oil well to production. It would also be desirable to provide a method of reducing MSPEP concentrations which is both practical and economical.